Centrifugal fan

ABSTRACT

A centrifugal fan includes an impeller including a blade portion having plural elongated blades arranged at a predetermined pitch in the circumferential direction and a motor for rotating the impeller. The impeller and the motor are arranged side by side in the axial direction. A diameter of the impeller is less than or equal to 25 millimeters. A radius r of the impeller and a length h in the axial direction of the impeller satisfies the relationship of 2r&lt;=h&lt;=20r. Among elements that constitute the motor, a torque generating portion including an armature and a field magnet, and a bearing portion that retains a rotational member in a rotatable manner to a fixed member are arranged side by side in the axial direction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a centrifugal fan for cooling embedded in electronic equipment and a motor thereof.

2. Description of the Prior Art

Recently, cooling fans embedded in electronic equipment such as a personal computer have become smaller and thinner. Conventionally, most electronic equipment incorporates a cooling fan of an axial flow type. It is because the axial flow type cooling fan typically has a structure suitable for a thin shape or a low profile. In contrast, although centrifugal fans have an advantage of having higher static pressure than axial fans, they also have a disadvantage of difficulty in reducing a dimension in the axial direction for a low profile compared with axial fans.

However, it is possible to reduce a dimension in the radial direction of an impeller of a centrifugal fan while increasing a length in the axial direction of the same and rotating the impeller at a high speed, so as to realize a centrifugal fan that can be embedded in low profile electronic equipment such as a note type personal computer.

FIG. 4 is a cross sectional view in the axial direction showing a structure of a centrifugal fan. In addition, FIG. 5 is a cross sectional view in the direction perpendicular to the rotational axis of an impeller in a typical centrifugal fan. Although shapes and scales of each element are not identical between FIG. 4 and FIG. 5, elements having the same functions are denoted by the same reference numeral. The centrifugal fan shown in FIG. 4 includes a housing 101 having a substantially cylindrical shape elongated in the axial direction, which houses an impeller 102 and a motor 103 for rotating the impeller 102. The impeller 102 is disposed mainly at the top side (the right side in FIG. 4) in the axial direction within the housing 101, while the motor 103 is disposed at the bottom (base) side (the left side in FIG. 4) in the axial direction within the housing 101.

The impeller 102 includes a blade portion 104 (at the top side) having plural blades 104 a of a shape elongated in the axial direction that are arranged at a predetermined pitch in the circumferential direction and a bottom end portion 105 having a substantially cylindrical shape for supporting the blade portion 104. The tip of the blade portion 104 is provided with a ring-like linking portion 106 for linking and supporting tip portions of the plural blades 104 a. When the impeller 102 rotates, external air is taken in through a suction opening 107 provided to the tip portion in the axial direction of the housing 101 as illustrated by an arrow IN. Then, the air is blown out through an outlet opening (corresponding to 108 in FIG. 5) externally that is provided to a part in the circumferential direction of the housing 101 (as illustrated by an arrow OUT in FIG. 5).

The motor 103 includes a rotational shaft 110, a sleeve 111 that constitutes a bearing of the rotational shaft 110, a sleeve holder 113, a base portion 114, an armature 115 that is a stator, a magnet 116 that is a rotator and a rotor yoke 117. The bottom end portion of the rotational shaft 110 abuts a thrust plate 118 that constitutes a thrust bearing. The tip portion of the rotational shaft 110 fits in and engages the rotor yoke 117, while the outer surface of the rotor yoke 117 at the top side fits in the inner surface of the impeller 102 at the bottom portion 105. A plurality of rotor magnets 116 is arranged in the circumferential direction on the inner surface of the rotor yoke 117 at the proximal end side. Therefore, the rotor magnet 116, the rotor yoke 117, the rotational shaft 110 and the impeller 102 rotate as one unit body.

The stator armature 115 is arranged on the outer surface of the sleeve holder 113 so that they are opposed to the rotor magnets 116 with a constant radial gap. When the stator armature 115 is driven (excited) to generate a revolving magnetic field, the rotor magnet 116, the rotor yoke 117, the rotational shaft 110 and the impeller 102 mentioned above are rotated as one unit body responding to the revolving magnetic field. Then, as described above, the rotation of the impeller 102 causes air flow that is taken in through the suction opening 107 at the tip portion in the axial direction of the housing 101 and is blown out through an outlet opening formed at a part in the circumferential direction of the housing 101.

It is required to design a cooling fan in a thinner shape for supporting low profile electronic equipment such as a note type personal computer that has a tendency of becoming thinner and thinner. In order to satisfy such a requirement by a centrifugal fan having the above-mentioned structure, it is necessary to reduce the dimension in the radial direction of the cylindrical outer shape. Although blowing capacity drops as the diameter of the impeller is decreased, it is possible to compensate the drop of the blowing capacity by increasing a length of the impeller in the axial direction and by increasing a rotation speed of the impeller.

However, it is difficult to reduce the dimension of the motor in the radial direction if the conventional structure is maintained. Namely, since it is necessary to rotate the impeller at a high speed and a predetermined torque for securing necessary blowing capacity, there is a limitation in downsizing the stator armature or the rotor magnet. In addition, there is also a limitation in reducing dimensions in the radial direction of the rotational shaft, the rotor yoke, the sleeve holder and the like for obtaining stable rotation of the impeller.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a centrifugal fan having a novel structure that enables further reduction of dimensions in the radial direction.

According to a first aspect of the present invention, a centrifugal fan is provided that includes an impeller including a blade portion having plural elongated blades arranged at a predetermined pitch in the circumferential direction and a motor for rotating the impeller. The impeller and the motor are arranged side by side in the axial direction. A diameter of the impeller is less than or equal to 25 millimeters. A radius r of the impeller and a length h in the axial direction of the impeller satisfies the relationship of 2r<=h<=20r. Among elements that constitute the motor, a torque generating portion including an armature and a rotor magnet, and a bearing portion that retains a rotational member in a rotatable manner to a stational member are arranged side by side in the axial direction.

As the impeller has a shape (elongated shape) as described above, a centrifugal fan is realized that can be embedded in low profile equipment. In addition, since it is a centrifugal fan, it has higher static pressure than an axial fan and is suitable for being embedded in compact electronic equipment of a high density of mounting components. In addition, as the torque generating portion and the bearing portion of the motor are arranged tandem in the axial direction, it is possible to reduce an outer diameter of the motor. Namely, if the torque generating portion and the bearing portion are arranged substantially at the same position in the axial direction (arranged in the radial direction), an outer diameter of the motor depends on a total sum of the outer diameter of the bearing portion and a size (thickness) of the torque generating portion (each member composing the motor) in the radial direction. In contrast, if the torque generating portion and the bearing portion are arranged tandem in the axial direction, an outer diameter of the motor depends on either the outer diameter of the bearing portion or the size (outer diameter) of the torque generating portion that is larger one than the other. Therefore, it is possible to reduce the outer diameter of the motor, thereby reducing further the dimension in the radial direction of the centrifugal fan.

According to a second aspect of the present invention, the impeller of the centrifugal fan is rotated at a rotation speed more than or equal to 10,000 revolutions per minute. Such a high rotation speed of the impeller secures sufficient quantity of air by the centrifugal fan with a reduced dimension (in the radial direction in particular).

According to a third aspect of the present invention, the rotational member rotates integrally as one element; the bearing portion is disposed at a barycenter of the element or at vicinity of the barycenter. Adopting this structure, rotation of the impeller can be stabilized easily. Namely, vibration of the impeller accompanying rotation thereof can be reduced, and a load to the bearing portion can be suppressed so that a life of the bearing portion can be increased. This structure is useful effective particularly in the case the impeller is rotated at a high rotation speed. Note that if the bearing portion is a pair of bearings arranged separately in the axial direction, the structure that the bearing portion is disposed at a barycenter of the element or at vicinity of the barycenter means a structure that the midpoint between the bearings is disposed at a barycenter of the element or at vicinity of the barycenter.

According to a fourth aspect of the present invention, an outer diameter of the blade portion of the impeller is substantially the same as an outer diameter of the torque generating portion of the motor. Adopting such a structure, it is possible to utilize a limited space efficiently when assembling the impeller and the motor in the housing or directly in electronic equipment.

According to a fifth aspect of the present invention, the motor is a DC brushless motor. A DC brushless motor has advantages of long life and low operating noise compared with a motor having brushes, and these advantages of the motor become advantages of the centrifugal fan.

According to a sixth aspect of the present invention, the bearing portion is a pair of ball bearings that are arranged separately in the axial direction. Using ball bearings, a bearing loss can be reduced, so that a centrifugal fan having a high efficiency can be realized.

According to a seventh aspect of the present invention, the pair of ball bearings are preloaded by pressing inner ring portions or outer ring portions of the ball bearings in the opposite direction to each other along the axial direction. When the ball bearing is preloaded, rattling thereof is suppressed so that more stable rotation can be obtained. As a method of preloading, either a constant pressure method or a constant position method can be adopted. The former method is performed by utilizing a spring member for giving a pressure. The latter method is performed by restricting movement of both the inner ring portion and the outer ring portion in the axial direction after preloading them.

According to an eighth aspect of the present invention, the bearing portion is a sleeve bearing including a rotational shaft that is engaged to the rotational member and a sleeve having a cylindrical shape that engages the outer surface of the rotational shaft with a small radial gap. In addition, a sleeve holder is provided that includes a cylindrical portion engaging the outer surface of the sleeve and a supporting portion extending from the center of a bottom end of the cylindrical portion to a base portion, and a stator armature constituting the torque generating portion is fixed to the outer surface of the supporting portion of the sleeve holder.

Adopting such a structure, a sleeve bearing that has lower cost and simpler structure than the ball bearing can be used for realizing the above-mentioned structure for further reducing an outer diameter of the centrifugal fan, in other words, the structure in which the torque generating portion and the bearing portion are arranged in the axial direction. Note that the same effect can be obtained by the following variation of the sleeve bearing. Namely, a fixed shaft is fixed to the stational member integrally, and the cylindrical sleeve engaging the outer surface of the fixed shaft with a radial gap is attached to the rotational member, so as to constitute a sleeve bearing. In this case, the stator armature constituting the torque generating portion is fixed to the base of the fixed shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view showing a structure of a centrifugal fan according to a first embodiment of the present invention.

FIG. 2 is a partial enlarged cross sectional view showing a detailed structure of a bearing portion in the first embodiment.

FIG. 3 is a cross sectional view showing a structure of a centrifugal fan according to a second embodiment of the present invention.

FIG. 4 is a cross sectional view in the axial direction showing a structure of a centrifugal fan.

FIG. 5 is a cross sectional view in the direction perpendicular to the axis of a housing and an impeller in a typical centrifugal fan.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described in detail with reference to attached drawings. Note that when referring to a relationship of positions or directions of structural elements in the following description, they mean a relationship of positions or directions in a drawing and do not mean a relationship of positions or directions in a state being incorporated in actual equipment.

Embodiment 1

FIG. 1 is a cross sectional view showing a structure of a centrifugal fan according to a first embodiment of the present invention. This centrifugal fan includes a housing 11 having a substantially cylindrical shape elongated in the axial direction, which houses an impeller 12 and a motor 13 for rotating the impeller 12. The impeller 12 is disposed at the top side (the right side in FIG. 1) in the axial direction within the housing 11, while the motor 13 is disposed at the bottom side (the left side in FIG. 1) in the axial direction within the housing 11.

The impeller 12 includes a blade portion 14 (at the top side) having plural blades of a shape elongated in the axial direction that are arranged at a predetermined pitch in the circumferential direction, and a bottom end portion 15 having a substantially cylindrical shape for supporting the blade portion 14. The tip of the blade portion is provided with a ring-like linking portion 16 for linking and supporting tip portions of the plural blades. When the impeller 12 rotates, external air is taken in through a suction opening 17 provided to the top end portion in the axial direction of the housing 11 as illustrated by an arrow IN. Then, the air is blown out through an outlet opening (corresponding to 108 in FIG. 5) externally that is provided to a part in the circumferential direction of the housing 11 (as illustrated by an arrow OUT in FIG. 5).

The impeller 12 having a shape elongated in the axial direction has a diameter less than or equal to 25 millimeters. In addition, a radius r and a length h in the axial direction of the impeller satisfies the relationship of 2r<=h<=20r. Since the impeller 12 has such an elongated shape, a centrifugal fan is realized that can be incorporated in low profile equipment. In addition, since it is a centrifugal fan, it has higher static pressure than an axial fan and is suitable for being embedded in compact electronic equipment of a high density of mounting components. In addition, the impeller 12 is rotated at a rotation speed more than or equal to 10,000 revolutions per minute, more preferably at a rotation speed more than or equal to 15,000 revolutions per minute. Such a high rotation speed of the impeller 12 secures sufficient quantity of air by the centrifugal fan with a reduced dimension (in the radial direction in particular).

The motor 13 has a structure in which a torque generating portion 13 a including a stator armature 21 and a rotor magnet 22 that is a field magnet. A bearing portion 13 b that retains a rotational member (a rotor yoke 25) in a rotatable manner are arranged in the axial direction with the torque generating portion 13 a. According to this structure, it is possible to reduce an outer diameter of the motor 13, therefore to reduce a dimension in the radial direction of the centrifugal fan.

In particular, according to this embodiment, the bearing portion 13 b is disposed at the top side (the impeller side) in the axial direction, while the torque generating portion 13 a is disposed at the bottom side. Namely, the bearing portion 13 b is disposed between the impeller 12 and the torque generating portion 13 a. More specifically, when plural members rotating as one unit body including the impeller 12 and the rotational member (the rotor yoke 25) are regarded as one element, the bearing portion 13 b is disposed at a barycenter of the element or at vicinity of the barycenter. According to this structure, rotation of the impeller can be stabilized easily. Namely, vibration of the impeller accompanying rotation thereof can be reduced, and a load to the bearing portion can be suppressed so that a life of the bearing portion can be increased. This structure is effective particularly in the case where the impeller 12 is rotated at a high rotation speed as described above.

The motor 13 in this embodiment is a DC brushless motor, and the stator armature 21 of the torque generating portion 13 a is arranged on the outer surface of the fixed shaft 23 that is a stational member. The fixed shaft 23 made of a metal is fit in a center through-hole of a base portion 24 made of a resin (or a metal) and is fixed to the same, and the base portion 24 is fixed to an inner surface at the bottom side of the housing 11 made of a metal. The base portion can be made of metal integrally with the fixed shaft. Thus, the fixed shaft 23 extends along the center axis of the substantially cylindrical housing 11 from the bottom side toward the top side by approximately a half of the length in the axial direction as shown in FIG. 1. Concerning a fixing method of the stator armature 21 with the fixed shaft 23, a fit-in (press fitting) method is adopted similarly to the fixing of the base portion 24 with the fixed shaft 23. An adhesive may further be used for fixing after the press fitting.

The rotor magnet 22 of the torque generating portion 13 a is fixed to the inner surface of the rotor yoke 25 that is a rotational member. For example, plural rotor magnets 22 are arranged on the inner surface of the rotor yoke 25 made of a magnetic material formed in a substantially cylindrical shape at a predetermined pitch in the circumferential direction, so that the rotor magnets 22 and the stator armatures 21 are radially opposed with a constant gap. It is preferable that the rotor magnet is made of a ring-like magnet.

The substantially cylindrical rotor yoke 25 is elongated in the axial direction and extends from the torque generating portion 13 a to the bearing portion 13 b. As understood from the cross section shown in FIG. 1, the rotor yoke 25 is provided with a step portion so that diameter decreases a little from the bottom end side to the top end side. The rotor magnet 22 is fixed on the inner surface of a large diameter portion 25 a at the bottom end side, while each outer ring portion of a pair of ball bearings 26 that constitute the bearing portion 13 b is fixed to the inner surface of the small diameter portion 25 b at the top end side. The ball bearings 26 are arranged separately in the axial direction, and the outer ring portions of them are fixed to the inner surface of the small diameter portion 25 b of the rotor yoke 25 by press fitting or loose fitting. Also in this case, an adhesive may further be used for fixing after the fitting. In addition, the bottom end portion 15 of the impeller 12 is fixed to the outer surface of the small diameter portion 25 b of the rotor yoke 25 by press fitting. Therefore, the rotor yoke 25 and the impeller 12 are rotated as one unit element.

FIG. 2 is a partial enlarged cross sectional view showing a detailed structure of a bearing portion in the first embodiment. The pair of ball bearings 26 (a ball bearing 26P at the bottom side and a ball bearing 26D at the top end side) constitutes the bearing portion 13 b. Each of the ball bearings 26 (26P or 26D) has a structure including an outer ring portion 26 a, an inner ring portion 26 b and plural balls 26 c that can roll freely between them. As described above, the outer ring portion 26 a of each ball bearing 26 is fixed to the inner surface of the small diameter portion 25 b of the rotor yoke 25. In addition, the inner ring portion 26 b engages the outer surface of the fixed shaft 23 in a slidable manner with a small clearance.

In the structure shown in FIG. 2, a stopper ring 27 is attached to the fixed shaft 23, and the stopper ring 27 prevents the inner ring portion 26 b of the ball bearing 26P at the bottom side from moving to the bottom side in the axial direction. In addition, a compressed coil spring 29 is disposed between the stopper ring 28 that is attached to the tip portion of the fixed shaft 23 and the inner ring portion 26 b of the ball bearing 26D at the top side. This compressed coil spring 29 preloads the inner ring portion 26 b of the ball bearing 26D at the top side toward the bottom side in the axial direction as shown by an arrow in FIG. 2. In addition, the outer ring portions 26 a of the ball bearings 26D and 26P at the distal end side and the proximal end side are fixed to the rotor yoke 25, while the movement of the inner ring portion 26 b of the ball bearing 26P at the bottom side to the bottom side in the axial direction is prevented by the stopper ring 27. Therefore, the inner ring portion 26 b of the ball bearing 26P at the bottom side is preloaded toward the top side in the axial direction as a reaction from the stopper ring 27.

In other words, the compressed coil spring 29 that is pressing means preloads the inner ring portions 26 b of the pair of ball bearings 26 inward in the axial direction. As a result, each of the ball bearings 26 is preloaded, so that the rotor yoke 25 as the rotational member and the impeller 12 that is fixed to the rotor yoke 25 can be rotated with stability.

As a variation of the structure shown in FIG. 2, a compressed coil spring a pressing means may be disposed between the inner ring portion 26 b of the ball bearing 26P at the bottom side and the inner ring portion 26 b of the ball bearing 26D at the top side. This compressed coil spring 29 can preload the pair of inner ring portions 26 b outward in the axial direction. Namely, the inner ring portion 26 b of the ball bearing 26P at the bottom side is pressed toward the bottom side in the axial direction, while the inner ring portion 26 b of the ball bearing 26D at the top side is pressed toward the top side in the axial direction. As a result, each of the ball bearings 26 is preloaded, so that the rotor yoke 25 as the rotational member and the impeller 12 that is fixed to the rotor yoke 25 can be rotated with stability.

As described above, in the structure shown in FIG. 2, the bearing portion 13 b includes the pair of ball bearings 26, the outer ring portions 26 a of the ball bearings 26 are fixed to the rotational member, the inner ring portions 26 b thereof engage the fixed shaft with a clearance, and pressing means (a compressed coil spring) are provided for preloading to the ball bearings 26 by pressing the pair of inner ring portions 26 b oppositely in the axial direction. Even if the above-mentioned structure of the bearing portion adopted, a dimension in the radial direction can be reduced sufficiently because the bearing portion 13 b is disposed not within the torque generating portion 13 a but at the position shifted from it in the axial direction in the centrifugal fan of the present invention. In addition, in either structure, a lubricant such as grease having high viscosity may be supplied between the inner ring portion 26 b and the fixed shaft 23, so that operation noise due to friction between them can be suppressed.

As another variation of this embodiment in which ball bearings are adopted as the bearing portion, a so-called constant position preload may be adopted instead of above-mentioned preloading using pressing means such as the compressed coil spring 29 (a so-called constant pressure preload). Namely, the pair of inner ring portion 26 b is pressed oppositely in the axial direction so as to preload the pair of ball bearings 26, and in this state the pair of inner ring portions 26 b is fixed to the fixed shaft 23 by means of adhesive or the like. This constant position preload method is advantageous in a simple structure and low cost compared with the constant pressure preload method using pressing means such as the compressed coil spring 29. However, the constant pressure preload method using pressing means is usually superior in durability and stability of the bearing.

In the centrifugal fan having the above-mentioned structure, when the stator armature 21 is driven (excited) to generate a revolving magnetic field, the rotor magnet 22, the rotor yoke 25 and the impeller 12 are rotated as one unit body responding to the revolving magnetic field. Then, the rotation of the impeller 12 causes air flow that is taken in through a suction opening 17 provided to the tip portion in the axial direction of the housing 11 and is blown out through an outlet opening that is provided to a part in the circumferential direction of the housing 11.

Embodiment 2

FIG. 3 is a cross sectional view showing a structure of a centrifugal fan according to a second embodiment of the present invention. This centrifugal fan includes a housing 11 having a substantially cylindrical shape elongated in the axial direction, which houses an impeller 12 and a motor 13 for rotating the impeller 12. The impeller 12 is disposed at the top side (the right side in FIG. 3) in the axial direction within the housing 11, while the motor 13 is disposed at the bottom side (the left side in FIG. 3) in the axial direction within the housing 11.

The impeller 12 includes a blade portion 14 (at the top side) having plural blades of a shape elongated in the axial direction that are arranged at a predetermined pitch in the circumferential direction, and a bottom portion 15 having a substantially cylindrical shape for supporting the blade portion 14. The tip of the blade portion is provided with a ring-like linking portion 16 for linking and supporting tip portions of the plural blades. When the impeller 12 rotates, external air is taken in through a suction opening 17 provided to the tip portion in the axial direction of the housing 111 as illustrated by an arrow IN. Then, the air is blown out through an outlet opening externally that is provided to a part in the circumferential direction of the housing 11.

The motor 13 has a structure in which a torque generating portion 13 a including a stator armature 21 and a rotor magnet 22, and a bearing portion 13 b for retaining a rotational member in a rotatable manner are arranged in the axial direction. In particular, according to this embodiment, the bearing portion 13 b is disposed at the top side (the impeller side) in the axial direction, while the torque generating portion 13 a is disposed at the bottom side. Although the ball bearings are used as the bearing portion 13 b in the first embodiment, a sleeve bearing that utilizes an oil impregnated metal or the like is used in the second embodiment. Namely, the sleeve bearing includes a sleeve 31 made of a cylindrical sintered metal impregnated with lubricating oil and a sleeve holder 32 that supports the sleeve 31.

The sleeve holder 32 that is made of a metal includes a cylindrical portion 32 a for engaging the outer surface of the sleeve 31 to support the same and a supporting portion 32 b extending from the center of a bottom end of the cylindrical portion 32 a to the bottom side. The axis of the cylindrical portion 32 a coincides with the axis of the shaft portion 32 b. The shaft portion 32 b is fit in the stator armature 21, and further the bottom end portion thereof is fixed to the center through-hole of the base portion 24 made of a resin (or a metal) by press fitting. The base portion 24 is fixed to an inner surface at the bottom side of the housing 11 made of a resin (or a metal). Thus, the sleeve holder 32 is fixed so that the axis of the sleeve holder 32 coincides with the axis of the housing 11. It is possible to fix the sleeve holder 32 to the stator armature 21 and the base portion 24 by using an adhesive after the press fitting.

The cylindrical portion 32 a of the sleeve holder 32 is attached so that the inner surface thereof engages the sleeve 31 made of a sintered metal impregnated with lubricating oil, and a rotational shaft 33 made of a metal is supported by the sleeve 31 in a rotatable manner. The top side of the rotational shaft 33 is fixed to a small diameter portion 25 c of a rotor yoke 25 that is the rotational member by press fitting. The rotor yoke 25 having a substantially cylindrical shape is elongated in the axial direction extending from the torque generating portion 13 a to the bearing portion 13 b. As understood from the cross section shown in FIG. 3, the rotor yoke 25 is provided with a step portion so that a diameter decreases from the bottom side to the top side in three steps. In other words, the rotor yoke 25 includes a large diameter portion 25 a, a medium diameter portion 25 b and a small diameter portion 25 c.

For example, a plurality of rotor magnets 22 are arranged on the inner surface of the large diameter portion 25 a of the rotor yoke 25 at a predetermined pitch in circumferential direction so that the rotor magnets 22 and the stator armature 21 are opposed with a constant gap. The bottom end portion 15 of the impeller 12 is engaged with and fixed to the outer surface of the medium diameter portion 25 b of the rotor yoke 25. The rotational shaft 33 is fixed to the small diameter portion 25 c of the rotor yoke 25 by press fitting as described above. Therefore, when the stator armature 21 is driven (excited) to generate a revolving magnetic field, the rotor magnet 22, the rotor yoke 25, the rotational shaft 33 and the impeller 12 are rotated as one unit body responding to the revolving magnetic field. Then, the rotation of the impeller 12 causes air flow that is taken in through a suction opening 17 provided to the tip portion in the axial direction of the housing 11 and is blown out through an outlet opening that is provided to a part in the circumferential direction of the housing 11.

In the cylindrical portion 32 a of the sleeve holder 32 that constitutes the sleeve bearing, a thrust plate 34 made of a metal (or a polymer) which has a preferable property of sliding is disposed at the inner surface at the bottom side so that the bottom end portion of the rotational shaft 33 can abut the thrust plate 34. This structure constitutes a thrust bearing. In addition, a seal member is provided for sealing the ring-like gap between an opening of the sleeve holder 32 at the top side and the rotational shaft 33. This seal member prevents dust from entering the cylindrical portion 32 a of the sleeve holder 32.

Also in this structure of the second embodiment in which the sleeve bearing is used bearing portion, the torque generating portion 13 a and the bearing portion 13 b can be arranged in the axial direction according to the present invention so that an outer diameter of the centrifugal fan can be reduced more. In addition, it is preferable that an outer diameter of the cylindrical portion 32 a of the sleeve holder 32 is smaller than an outer diameter of the stator armature 21 as shown in FIG. 3. According to this structure, the motor 13 can be assembled easily while securing a sufficient size of the stator armature 21 for obtaining a sufficient driving torque of the motor 13. In addition, an outer diameter of the shaft portion 32 b of the sleeve holder 32 is larger than an outer diameter of the rotational shaft 33. According to this structure, stiffness of the sleeve holder 32 is increased so that the impeller 12 can be rotated with stability.

Although some embodiments and variations of the present invention are described above, the present invention can be embodied in various ways without limited to these embodiments and variations. In addition, materials and shapes of each members mentioned in the above description are merely embodiments, and it should not be interpreted that structures of the present invention are limited to those materials and shapes.

For example, the sleeve in the second embodiment may be attached to the rotor yoke 25 that is the rotational member, and a fixed shaft 23 is fixed to the base 24 as one body or separated bodies. The sleeve bearing of this structure is particularly suitable for downsizing.

While embodiments of the present invention have been shown and described, it will be understood that the present invention is not limited thereto, and that various changes and modifications may be made by those skilled in the art without departing from the scope of the invention as set forth in the appended claims and their equivalents. 

1. A centrifugal fan comprising: an impeller including a blade portion having plural elongated blades arranged at a predetermined pitch in the circumferential direction, said blade portion having a radius r and a length h in the axial direction of the impeller with satisfying a relationship 2r<=h<=20r; a rotational member including said impeller; a rotor magnet secured to said rotational member; a stator armature which composes a stational member; a torque generating portion including said stator armature and said rotor magnet; a bearing portion which retains said rotational member in a rotatable manner to said stational member arranged tandem in the axial direction with said torque generating portion; and a motor including said rotor, said stator, said torque generating portion and said bearing portion.
 2. The centrifugal fan according to claim 1, wherein the rotational speed of the impeller is set more than or equal to 10,000 revolutions per minute.
 3. The centrifugal fan according to claim 1, wherein said rotational member rotates as one element; and the bearing portion is disposed at a barycenter of the element or at vicinity of the barycenter of the same.
 4. The centrifugal fan according to claim 2, wherein said rotational member rotates as one element; and the bearing portion is disposed at a barycenter of the element or at vicinity of the barycenter of the same.
 5. The centrifugal fan according to claim 1, wherein an outer diameter of the blade portion of the impeller is substantially the same as an outer diameter of the torque generating portion of the motor.
 6. The centrifugal fan according to claim 2, wherein an outer diameter of the blade portion of the impeller is substantially the same as an outer diameter of the torque generating portion of the motor.
 7. The centrifugal fan according to claim 1, wherein the motor is a DC brushless motor.
 8. The centrifugal fan according to claims 1, wherein the bearing portion is a pair of ball bearings that are arranged separately in the axial direction.
 9. The centrifugal fan according to claims 2, wherein the bearing portion is a pair of ball bearings that are arranged separately in the axial direction.
 10. The centrifugal fan according to claim 8, wherein the pair of ball bearings are preloaded by pressing inner ring portions or outer ring portions of the ball bearings in the opposite direction to each other along the axial direction.
 11. The centrifugal fan according to claim 1, wherein the bearing portion is a sleeve bearing including a rotational shaft that is connected to the rotational member; a sleeve having a cylindrical shape that engages the outer surface of the rotational shaft with a minute gap, and the centrifugal fan further comprises a cylindrical portion in which the sleeve is held; a supporting portion that retains the cylindrical portion to a base portion; a sleeve holder that includes the cylindrical portion and the supporting portion; a stator armature constituting the torque generating portion which is fixed to the outer surface of the supporting portion of the sleeve holder.
 12. The centrifugal fan according to claim 2, wherein the bearing portion is a sleeve bearing including a rotational shaft that is connected to the rotational member; a sleeve having a cylindrical shape that engages the outer surface of the rotational shaft with a minute gap, and the centrifugal fan further comprises a cylindrical portion in which the sleeve is held; a supporting portion that retains the cylindrical portion to a base portion; a sleeve holder that includes the cylindrical portion and the supporting portion; a stator armature constituting the torque generating portion which is fixed to the outer surface of the supporting portion of the sleeve holder.
 13. The centrifugal fan according to claim 3, wherein the bearing portion is a sleeve bearing including a rotational shaft that is connected to the rotational member; a sleeve having a cylindrical shape that engages the outer surface of the rotational shaft with a minute gap, and the centrifugal fan further comprises a cylindrical portion in which the sleeve is held; a supporting portion that retains the cylindrical portion to a base portion; a sleeve holder that includes the cylindrical portion and the supporting portion; a stator armature constituting the torque generating portion which is fixed to the outer surface of the supporting portion of the sleeve holder.
 14. The centrifugal fan according to claim 1, wherein the bearing portion is a sleeve bearing including a fixed shaft that is fixed to the stational member and a sleeve having a cylindrical shape that engages the outer surface of the fixed shaft with a radial minute gap, and a stator armature constituting the torque generating portion is fixed to the outer surface of the fixed shaft.
 15. The centrifugal fan according to claim 2, wherein the bearing portion is a sleeve bearing including a fixed shaft that is fixed to the stational member and a sleeve having a cylindrical shape that engages the outer surface of the fixed shaft with a radial minute gap, and a stator armature constituting the torque generating portion is fixed to the outer surface of the fixed shaft.
 16. The centrifugal fan according to claim 3, wherein the bearing portion is a sleeve bearing including a fixed shaft that is fixed to the stational member and a sleeve having a cylindrical shape that engages the outer surface of the fixed shaft with a radial minute gap, and a stator armature constituting the torque generating portion is fixed to the outer surface of the fixed shaft. 